Rainbow Electronics MAX8668 - Manual (Page 14)

MAX8667/MAX8668
1.5MHz Dual Step-Down DC-DC Converters
with Dual LDOs and Individual Enables
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for peak ripple current and load transients. The step-
down converter’s unique architecture has minimal cur-
rent overshoot during startup and load transients and in
most cases, an inductor capable of 1.3x the maximum
load current is acceptable.
For output voltages above 2V, when light-load efficiency
is important, the minimum recommended inductor is
2.2µH. For optimum voltage-positioning load transients,
choose an inductor with DC series resistance in the
50mΩ to 150mΩ range. For higher efficiency at heavy
loads (above 200mA) and minimal load regulation,
keep the inductor resistance as small as possible. For
light-load applications (up to 200mA), higher resistance
is acceptable with very little impact on performance.
Capacitor Selection
Input Capacitors
The input capacitor for the step-down converters (C2 in
Figures 3 and 4) reduces the current peaks drawn from
the battery or input power source and reduces switch-
ing noise in the IC. The impedance of C2 at the switch-
ing frequency should be very low. Surface-mount
ceramic capacitors are a good choice due to their
small size and low ESR. Make sure the capacitor main-
tains its capacitance over temperature and DC bias.
Ceramic capacitors with X5R or X7R temperature char-
acteristics generally perform well. A 10µF ceramic
capacitor is recommended.
A 4.7µF ceramic capacitor is recommended for the
LDO input capacitor (C3 in Figure 3).
Step-Down Output Capacitors
The step-down output capacitors (C6 and C7 in Figures
3 and 4) are required to keep the output-voltage ripple
small and to ensure regulation loop stability. These
capacitors must have low impedance at the switching
frequency. Surface-mount ceramic capacitors are a
good choice due to their small size and low ESR. Make
sure the capacitor maintains its capacitance over tem-
perature and DC bias. Ceramic capacitors with X5R or
X7R temperature characteristics generally perform well.
The output capacitance can be very low. For most
applications, a 2.2µF ceramic capacitor is sufficient.
For optimum load-transient performance and very low
output ripple, the output capacitor value in µF should
be equal to or greater than the inductor value in µH.
Feed-Forward Capacitor
The feed-forward capacitors on the MAX8668 (C4 and
C5 in Figure 4) set the feedback loop response, control
the switching frequency, and are critical in obtaining
the best efficiency possible. Small X7R and C0G
ceramic capacitors are recommended.
For OUT1, calculate the value of C4 as follows:
C4 = 1.2 x 10 -5 (s/V) x (V OUT / R1)
For OUT2, calculate the value of C5 and C10 as fol-
lows:
C ff = 1.2 x 10 -5 (s/V) x (V OUT / R3)
C ff = C5 + (C10 / 2)
(C10 / C5) + 1 = (V OUT / V FB ), where V FB is 0.6V.
Rearranging the formulas:
C10 = 2 x C ff x (V
OUT
- V
FB
)/(V
OUT
+ V
FB
)
C5 = C ff – (C10 / 2)
C10 is needed if V OUT > 1.5V or V IN12 can be less than
V OUT / 0.65.
MANUFACTURER INDUCTOR L (µH) R L (m Ω) CURRENT RATING (A) L x W x H (mm)
FDK MIPF2016 2.2 110 1.1 2.0 x 1.6 x 1.0
FDK MIPF2520D 2.2 80 1.3 2.5 x 2.0 x 1.0
LQH32CN2R2M5 2.2 97 0.79 3.2 x 2.5 x 1.55
Murata
LQM31P 2.2 220 0.9 3.2 x 1.6 x 0.95
Sumida CDRH2D09 2.2 120 0.44 3.2 x 3.2 x 1.0
TDK GLF251812T 2.2 200 0.6 2.5 x 1.8 x 1.35
TOKO D2812C 2.2 140 0.77 2.8 x 2.8 x 1.2
TOKO MDT2520-CR 2.2 80 0.7 2.5 x 2.0 x 1.0
TPC Series 2.2 55 1.8 4.0 x 4.0 x 1.1
Wurth
TPC Series 4.7 124 1.35 4.0 x 4.0 x 1.1
Taiyo Yuden CB2518T 2.2 90 0.51 2.5 x 1.8 x 2.0
Table 1. Recommended Inductors